Kinetics of polymerization of methyl methacrylate initiated by photolysis of benzil with 420 nm radiation, have been investigated at 35 °C. The rate of monomer disappearance has been followed gravimetrically and the chain-length of the polymer formed, measured by viscometry. The dependence of the rate of polymerization and quantum yield of monomer conversion on the concentrations of the monomer and the initiator, absorbed light-intensity etc. has been studied in detail. A kinetic scheme has been proposed in the light of experimental results, involving i) a primary photochemical act of excitation of the sensitizer producing free radicals by hydrogen abstraction, ii) initiation of polymerization by free radicals generated and iii) termination of the active chain by mutual combination involving either coupling or disproportionation. © 1978 Dr. Dietrich Steinkopff Verlag.